This invention relates to optical data processing systems. More particularly, it involves a data processing system in which the Fourier transform or spatial frequency function of an optical data source is optically derived.
Optical data processing systems which perform a Fourier transform analysis of an optical data source are well known in the art. See e.g., "Use of the Fourier Transformable Properties of Lenses for Signal Spectrum Analysis," by K. Preston, Jr., Chapter 4 of Optical and Electro-optical Information Processing, published by M.I.T. Press; and "Filtering Operations using Coherent Optics," by Cutrona et al, reprinted from Volume XV, Proceedings of the National Electronics Conference, Hotel Sherman, Chicago, Ill., Oct. 12-14, 1950. Typically, the data source includes a transparency having a developed image thereon of the data whose spatial frequency analysis is desired. In one application, the data source may include a transparency in which the inputs from a plurality of Very Large Array (VLA) radio telescopes are superimposed. In another application, the data source may be a picture of ocean waves where the wave frequency and direction are to be measured.
In any event, the spatial frequency function of the data source is supplied by optically deriving the Fourier transform of the data source. In the system, a source of coherent monochromatic light is collimated by a spherical lens and projected upon the data source which, as noted above, carries imprinted thereon a signal function consisting of density variations. The Fourier transform plane is disposed on the other side of the data source at the focal plane of a transform lens. The light passing through the data source will be diffracted by the density variations in the data source and will appear as spots at the Fourier transform plane. These spots will be spaced from the central DC beam depending upon the frequency of the input data. The DC beam, as it is commonly referred to, is representative of the zero frequency component of the data and can be envisioned as a line passing through the center of the light source, collimator lens, data source, transform lens, and Fourier transform plane. The higher frequency components of the input data are displaced proportionately from the DC beam at the Fourier transform plane according to their frequency, i.e., the higher frequency components being displaced further away from the DC beam.
Unfortunately, phase errors can be introduced into the system due to imperfections in the lenses, variations in the thickness of the data source transparency, etc. As is known in the art, such phase errors will disturb the accuracy of the optically derived Fourier transform. The use of precision components can minimize the phase error. However, the cost of such components may be prohibitive in many applications.
Therefore, this invention is directed broadly to compensating for phase errors in such optical data processing systems thereby permitting the use of components having less precision to reap the benefits of their inherently lower cost.